Michael Zharnikov

Light‐Powered Electrical Switch Based on Cargo‐Lifting Azobenzene Monolayers

Inspired by the complex molecular machines found in nature, chemists have developed much simpler molecular motors. Among them, several systems incorporating azobenzene have been proposed, which exploit the reversible trans–cis isomerization triggered by light or an electric field for applications such as optical data-storage devices, switchable supramolecular cavities, and sensors. Recently, it has been demonstrated that the photoisomerization process of individual polymer chains incorporating azobenzenes can express mechanical work. In light of these findings, one can foresee self-assembled monolayers (SAMs) of aromatic azobenzenes as molecular systems able to express forces of unprecedented magnitude by exploiting a collective subnanometer structural change. We recently designed a rigid and fully conjugated azobenzene exposing a thiol anchoring group, which was able to form a tightly packed SAM on Au(111) (SAMAZO). Scanning tunneling microscopy (STM) studies revealed that upon light irradiation of the chemisorbed SAMs, a collective isomerization of entire molecular-crystalline domains occurred with an outstandingly high directionality. Based on these results, a cooperative nature of the isomerization of adjacent AZO molecules has been proposed. Furthermore, the joint action of the molecules in the SAM provides an ideal system as a potential “cargo” lifter. Herein, we show that, upon irradiation, azobenzene SAMs incorporated in a junction between an Au(111) surface and a mercury drop are able to 1) lift the “heavy” Hg drop, and 2) reversibly photoswitch the current flowing through the junction (Figure 1). Current–voltage (I–V) characteristics averaged over more than 30 junctions incorporating AZO SAMs in the trans and the cis conformations are shown in Figure 2a. The SAMAZO in the cis conformation was obtained with extremely high yield (98%) upon irradiation by UV light of the SAMAZO initially formed by the trans conformer. The difference in the measured currents, which amounts to about 1.4 orders of magnitude, is in agreement with a through-bond tunneling mechanism described by Equation (1).

Cooperative light-induced molecular movements of highly ordered azobenzene self-assembled monolayers

Photochromic systems can convert light energy into mechanical energy, thus they can be used as building blocks for the fabrication of prototypes of molecular devices that are based on the photomechanical effect. Hitherto a controlled photochromic switch on surfaces has been achieved either on isolated chromophores or within assemblies of randomly arranged molecules. Here we show by scanning tunneling microscopy imaging the photochemical switching of a new terminally thiolated azobiphenyl rigid rod molecule. Interestingly, the switching of entire molecular 2D crystalline domains is observed, which is ruled by the interactions between nearest neighbors. This observation of azobenzene-based systems displaying collective switching might be of interest for applications in high-density data storage.

Electron-induced crosslinking of aromatic self-assembled monolayers: Negative resists for nanolithography

We have explored the interaction of self-assembled monolayers of 1,1′-biphenyl-4-thiol (BPT) with low energy electrons. X-ray photoelectron, infrared, and near edge x-ray absorption fine structure spectroscopy showed that BPT forms well-ordered monolayers with the phenyl rings tilted ∼15° from the surface normal. The films were exposed to 50 eV electrons and changes were monitored in situ. Even after high (∼10 mC/cm2) exposures, the molecules maintain their preferred orientation and remain bonded on the gold substrate. An increased etching resistance and changes in the infrared spectra imply a crosslinking between neighboring phenyl groups, which suggests that BPT can be utilized as an ultrathin negative resist. This is demonstrated by the generation of patterns in the underlying gold.

Modification of thiol-derived self-assembling monolayers by electron and x-ray irradiation: Scientific and lithographic aspects

This article reviews recent experiments on the modification of thiol-derived self-assembling monolayers (SAMs) by electron and x-ray irradiation. Several complementary experimental techniques such as near-edge x-ray absorption fine structure spectroscopy, x-ray photoelectron spectroscopy and microscopy, and infrared reflection absorption spectroscopy were applied to gain a detailed knowledge on the nature and extent of irradiation-induced damage in these systems. The reaction of a SAM to electron and x-ray irradiation was found to be determined by the interplay of the damage/decomposition and cross-linking processes. Ways to adjust the balance between these two opposing effects by molecular engineering of the SAM constituents are demonstrated. The presented data provide the physical–chemical basis for electron-beam patterning of self-assembled monolayers to extend lithography down to the nanometer scale.

Spectroscopic characterization of thiol-derived self-assembling monolayers

This article reviews recent progress in the spectroscopic characterization of aliphatic and aromatic thiol-derived self-assembled monolayers (SAMs) on noble metal substrates. Several complementary techniques such as near edge x-ray absorption fine structure spectroscopy, x-ray photoelectron spectroscopy, and infrared reflection absorption spectroscopy were applied to study the balance between intermolecular and adsorbate-substrate interactions, chemical identity of the headgroup, and absorption site homogeneity at the sulphur-metal interface. Whereas in the thioaliphatic SAMs the headgroup-substrate interaction was found to be a decisive factor for the structure and packing in these films, these parameters are mainly determined by the intermolecular interactions in the thioaromatic films. Only one sulphur species could be detected in the S 2p HRXPS spectra of both aliphatic and aromatic SAMs suggesting binding of individual molecules as thiolates. Conclusions on the heterogeneity of the adsorption sites are derived and evidence that the investigated films represent highly correlated molecular assemblies are presented.

Self-assembly of a pyridine-terminated thiol monolayer on Au(111).

Self-assembled monolayers (SAMs) of 3-(4-pyridine-4-yl-phenyl)-propane-1-thiol (PyP3) on Au(111)/mica have been studied by scanning tunneling microscopy (STM), polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS), high-resolution X-ray photoemission spectroscopy (HRXPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The quality of the SAM is found to be strongly dependent on the solvent. Substantial gold corrosion is observed if pure ethanol is used. In contrast, highly ordered and densely packed SAMs are formed from acetonitrile or a KOH/ethanol mixture. The structure is described by a 2 radical3 x radical3 unit cell with the aromatic moiety oriented nearly perpendicular to the surface. The PyP3 films form with the pyridine moiety deprotonated. Variation of pH allows reversible protonation without measurable damage of the SAM.

The effect of sulfur–metal bonding on the structure of self-assembled monolayers

The equilibrium structure of alkanethiol monolayers self-assembled on metal substrates is determined by a delicate interplay of the intermolecular chain–chain and chemisorptive substrate–head group inter actions. To understand the role of the individual constituents of this interplay, we studied the structure of biphenyl and perfluoroalkyl terminated alkanethiol monolayers self-assembled on Au and Ag. The structural characteristics of the monolayers derived from NEXAFS, FTIRRAS and XPS measurements point to a decisive role of the directional substrate–head group interactions.

Modification of alkanethiolate monolayers on Au-substrate by low energy electron irradiation: Alkyl chains and the S/Au interface

Low-energy electron irradiation damage in alkanethiol (AT) self-assembled monolayers (SAM) has been studied by using hexadecanethiolate [HDT: CH3–(CH2)15–S-] film on Au-substrate as a model system. The induced changes were monitored by insitu photoelectron spectroscopy and angle resolved near edge X-ray absorption fine structure spectroscopy. AT SAMs are found to be very sensitive to low-energy electron irradiation. Both the alkyl chains and the S/Au interface are affected simultaneously through the electron-induced dissociation of C–H, C–C, C–S, and Au–thiolate bonds. The most noticeable processes are the loss of the orientational and conformational order, partial dehydrogenation and desorption of the film, and the appearance of new sulfur species. The latter process can be related to the formation of disulfide at the S/Au interface or an incorporation of the thiolate (or the corresponding radical) into the alkyl matrix via bonding to irradiation-induced carbon radicals in the adjacent aliphatic chains. The most essential damage in the AT films occurs in the early stages of irradiation. Irradiation with a dose of 1000 µC cm-2 (about 13 electrons per HDT chain) at the primary electron energy of 50 eV results in almost complete breakdown of the orientational order in the initially well-ordered HDT film, a decrease of its thickness by about 25%, and a destruction of ≈40% of the original Au–thiolate bonds. The film becomes a disordered structure comprising both saturated and unsaturated hydrocarbons. Further irradiation of the residual film is accompanied by a continuous C–C bond cleavage and the desorption of the remaining hydrogen, which merely leads to increasing cross-linking and the transformation of saturated hydrocarbons into unsaturated ones through C2C double bond formation.

Impact of DNA–Surface Interactions on the Stability of DNA Hybrids

The structure and stability of single- and double-stranded DNA hybrids immobilized on gold are strongly affected by nucleotide-surface interactions. To systematically analyze the effects of these interactions, a set of model DNA hybrids was prepared in conformations that ranged from end-tethered double-stranded to directly adsorbed single-stranded (hairpins) and characterized by surface plasmon resonance (SPR) imaging, X-ray photoelectron spectroscopy (XPS), fluorescence microscopy, and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The stabilities of these hybrids were evaluated by exposure to a series of stringency rinses in solutions of successively lower ionic strength and by competitive hybridization experiments. In all cases, directly adsorbed DNA hybrids are found to be significantly less stable than either free or end-tethered hybrids. The surface-induced weakening and the associated asymmetry in hybridization responses of the two strands forming hairpin stems are most pronounced for single-stranded hairpins containing blocks of m adenine (A) nucleotides and n thymine (T) nucleotides, which have high and low affinity for gold surfaces, respectively. The results allow a qualitative scale of relative stabilities to be developed for DNA hybrids on surfaces. Additionally, the results suggest a route for selectively weakening portions of immobilized DNA hybrids and for introducing asymmetric hybridization responses by using sequence design to control nucleotide-surface interactions--a strategy that may be used in advanced biosensors and in switches or other active elements in DNA-based nanotechnology.

Making Protein Patterns by Writing in a Protein-Repelling Matrix

One of the challenges of modern nanotechnology is the development of reliable, efficient, and flexible methods for the fabrication of ordered and complex patterns of proteins. Such patterns are of importance for biology and medical science: examples are proteomics, panel immunoassays, cell research, pharmaceutical screening for potential drugs, medical diagnostics, and encoding directional biological information. An essential element of almost all the available techniques is a protein-repelling background matrix which surrounds the active protein-adsorbing areas and prevents adsorption of proteins beyond these areas. Such a matrix is usually comprised of oligoor poly(ethylene glycol)based materials, polymers, or self-assembled monolayers (SAMs), and is generally prepared by a backfilling procedure after the fabrication of the protein-attracting patterns. Herein we present an alternative approach, showing that the proteinrepelling films, both SAMand polymer-like, can be used as a primary matrix for direct electron-beam writing of both nonspecific and specific protein patterns of any shape, including gradient ones, on a variable length scale. These factors make the approach quite flexible, which is additionally strengthened by the intrinsic versatility of electron-beam lithography (EBL), a wide range of suitable electron energies, the broad availability of commercial oligoethylene glycol (OEG) compounds, variable substrate material, and the wide choice of the target proteins. The approach is schematically illustrated in Figure 1. We used protein-repelling SAMs of OEG-substituted alkanethiols, HO(CH2CH2O)n(CH2)11SH with n = 3 (EG3) and 7 (EG7), on evaporated Au(111) substrates. Generally, the first step (or steps) to fabricate a protein pattern is to prepare a SAM-based chemical template. Such templates can be made by a combination of direct writing (molecules with specific binding groups to attract or bind proteins or intermediate moieties) and backfilling (OEGbased molecules) as in microcontact printing or dip-pen lithography. 4, 7] In EBL, fabrication of a chemical template suitable for protein adsorption can be performed either by transformation of specific tail groups of an aromatic SAM or by the irradiation-promoted exchange reaction (IPER) between a primary aliphatic SAM and a molecular substituent. The transformation of specific SAM tail groups however, requires an additional exchange-reaction-mediated backfilling of non-irradiated areas by OEG-based molecules, which is a slow process. The possibilities of IPER are limited as well, because of its low efficiency in the case of long-chain OEG-based SAMs. Therefore, only inverse protein patterns (protein-repelling features on a protein-adsorbing background) have been fabricated by the IPER method to date. In view of these problems, patterning aliphatic SAMs directly, similar to the aromatic films, could be considered. However, in contrast to aromatic films, a tail group of an aliphatic SAM usually cannot be specifically modified by electron irradiation without severe damage to the entire film, which deteriorates the overall quality of the template. We found, however, that this behavior does not occur in the case of OEG-terminated SAMs. According to X-ray photoelectron spectroscopy (XPS) data (see Figure 2 a and Figure SI1 in the Supporting Information), the OEG part of such films is extremely sensitive to electron irradiation (similar behavior was previously observed for UV-light exposure). It is modified to a severe extent even at very low doses ( 1 mCcm ), but both the aliphatic part and thiolate anchor of the SAM remain mostly intact, maintaining a thorough coupling of the molecules to the substrate. As a result of the electron-induced decomposition of the OEG chain, the effective thickness of the OEG SAM progressively decreases in the course of irradiation (Figure 2b). Along with the thickness reduction, the cleavage of the C O bonds within the ethylene glycol (EG) units leads to the generation of chemically active sites for subsequent nonspecific binding of different moieties. The amount of adsorbate is governed by the density of these sites, that is, by the primary irradiation dose. As shown in Figure 2b (see also Figure SI2 in the Supporting Information), progressive irradiation of the EG7 and EG3 SAMs results in a progressive increase in protein affinity until saturation (an affinity which is 100 % that of a dodecanthiolate (DDT) SAM) at higher doses. Extensive adsorption of proteins occurs even at small thickness reduction, especially for EG3/Au, thus it is the newly formed chemically active sites that are responsible for the protein attachment and not “holes” in the primary film which occur during the thickness reduction. The selection of an appropriate dose allows a precise tuning of the protein coverage from zero to the values typical for surfaces with high protein affinity (DDT SAMs). By combining this approach with lithography, it is possible to fabricate any desired protein pattern, including gradientlike ones. An example is given in Figure 3a, where an AFM image of a gradient-like fibrinogen pattern surrounded by the [*] Dr. N. Ballav, Prof. Dr. M. Zharnikov Angewandte Physikalische Chemie Universit t Heidelberg, 69120 Heidelberg (Germany) Fax: (+ 49)6221-546-199 E-mail: michael.zharnikov@urz.uni-heidelberg.de